In the printing industry, the photosensitive planographic printing plate, e.g., an offset PS plate is now widespread, for instance, for commercial printing, news printing, form printing and carton printing, because of the simplicity of handling thereof and the elimination of labor in the process for producing a press plate and in printing comparing with a conventional wipe-on plate.
Recently, new process systems and automation systems for producing the planographic printing plate are proposed to meet the demand of further elimination of labor or high-speed processing, and so the photosensitive planographic printing plate which is highly sensitive and applicable to such new systems is strongly desired. Hitherto, for instance, the photosensitive planographic printing plate had been exposed by closely contacted with an original image-bearing film and irradiated with an actinic light to form an image pattern on the surface thereof. However, recently a new exposing system wherein the original image of micro film is enlarged and projected directly onto the surface of the photosensitive planographic printing plate to form the image is developed and made into practice. This new system is advantageous in comparison with the conventional process in view of the saving of silver film and the automatization of step and repeat exposure. Another new system wherein the photosensitive layer of the sensitized planographic printing plate is directly exposed by a scanning of visible ray of laser such as argon ion laser or ultra violet ray to form the image is also developed. Such laser scanning exposing system is very favorable, because the system saves much more labor due to no necessity to make the silver film and because the working of producing the original image will be increasingly computerized and much of original image will be expected to be directly produced as a signal from a computer.
To form the image pattern effectively by those new exposing systems, the photosensitive planographic printing plate is required with extremely high exposing speed comparing with the conventional.
Moreover, with respect to the development it is desirable that such photosensitive planographic printing plate can be developed in an aqueous alkaline solution, because the aqueous solution developer is more advantageous than the organic solvent developer in view of the safety or sanitation of the working circumstance and the developing cost.
Therefore, the object of this invention is to provide a photosensitive planographic printing plate having an extremely high exposing speed so as to be applicable to the above mentioned new exposing systems, and excellent in printing endurance and printability, and in addition capable of being developed in the aqueous alkaline solution.
For producing a highly sensitive planographic printing plate, the following conditions must be fully satisfied. (1)Firstly, the photosensitive composition which undergoes a chain photoreaction causing an amplification effect must be used, instead of the conventional photosensitive composition of photo-single-reaction type such as a diazo composition, e.g., a condensation product of para-diazo-diphenylamine and formaldehyde, or a photodimerizable photosensitive resin having .beta.-phenyl acrylate group on a main or side chain. (2)Secondly, the coating layer in the imaged area obtained by the photohardening can adhere strongly to the aluminium grained plate for ensuring the excellent printing endurance. (3)Thirdly, the photosensitive composition in the unexposed or non-imaged area can be dissolved fully by the developer and removed completely, and the bare surface of the aluminium grained plate is hardly stained with a printing ink or easily cleanable in case of stained, and has an excellent hydrophilic property resulting in the excellent printability.